Need help about starting an excercise routine?

OK I'm trying to start an exercise routine and I was wondering if it is better to first to shave some lbs and then start with the weight training or do them both at the same time. I'm planning to lose around 20lbs. So it will be slower if I do both, but I would look better all at once in the end. I'm pretty sure that conventional wisdom is to do both, but won't doing both psychologically deter me because I won't be shaving the lbs as fast? Thanks in advance for your input.